The goal of plant breeding is to develop new, unique, and superior cultivars and hybrids. A breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, to eventually produce many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing, and mutagenesis. Such a breeder has no direct control of the process at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same traits.
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools, from which cultivars are developed by selfing and selection of desired phenotypes. The new cultivars are evaluated to determine which have commercial potential. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include higher seed yield, resistance to diseases and insects, better stems and roots, tolerance to drought and heat, and better agronomic quality.
The choice of breeding and selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar and pureline cultivar). For highly heritable traits, a choice of superior individual plants evaluated at a single location may be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences the choice of the breeding and selection methods. For example, backcross breeding may be used to transfer one (or a few) favorable genes for a highly heritable trait into a desirable germplasm. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques may be used to improve quantitatively-inherited traits controlled by numerous genes.
A breeding program typically includes a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary, depending on the goal and objectives, but the criteria may include, for example and without limitation: gain from selection per year (based on comparisons to an appropriate standard); overall value of the advanced breeding lines; and the number of successful cultivars produced per unit of input (e.g., per year and per dollar expended).
Promising advanced breeding lines are then thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s), typically for three or more years. Candidates for new commercial cultivars are selected from among the best lines; those still deficient in a few traits may be used as parents to produce new populations for further selection. These processes, which lead to the final step of marketing and distribution, usually take from 8 to 12 years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
Breeding programs combine desirable traits from two or more inbred lines, or various broad-based sources, into breeding pools from which new inbred lines are developed by selfing and selection of desired phenotypes. A hybrid variety is the cross of two such inbred lines, each of which may have one or more desirable characteristics absent in one line, or complementing the other. The new inbred plants are crossed with other inbred lines, and the hybrids from these crosses are evaluated to determine which are superior, or possess desirable attributes. Hybrid seed is produced by manual crosses between selected male-fertile parents, or by using male sterility systems. These hybrids are selected for certain single gene traits (e.g., pod color, flower color, pubescence color, and herbicide resistance) that indicate that the seed is truly a hybrid. Data on parental lines, as well as the phenotype of the hybrid, influence the breeder's decision regarding whether to continue with the specific hybrid cross.
Accordingly, the development of new cultivars requires the selection of parent varieties, crossing of these varieties, and selection of superior hybrid crosses. The task of identifying genetically superior individuals is particularly difficult. One method of identifying a superior plant is to determine one or more phenotypes in the plant, for example, relative to other experimental plants and to a widely grown standard cultivar. This task is extremely difficult, because (for most traits) the true genotypic value is masked by other confounding plant traits or environmental factors. Thus, it is typically necessary to determine the precise genotype of a particular plant, and its phenotype, in order to adequately evaluate and identify superior cultivars and hybrids.
The composition of a particular plant cultivar developed during selective plant breeding is unpredictable. This unpredictability is due, in part, to the breeder's selection, which occurs in unique environments, and which allows no control at the DNA level (using conventional breeding procedures), with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. Similarly, the same breeder cannot produce the same cultivar twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of resources, monetary and otherwise, to develop superior new cultivars.
Pedigree breeding is used commonly for the improvement of self-pollinating crops. In pedigree breeding, two parents that possess favorable, complementary traits are crossed to produce F1 progeny. An F2 population is produced by selling one or several plants from the F1 progeny generation. Selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. To improve the effectiveness of selection for traits with low heritability, replicated testing of families can begin in the F4 generation. At an advanced stage of inbreeding (e.g., F6 or F7), the best lines or mixtures of lines with similar phenotypes are tested for potential release as new cultivars.
Mass and recurrent selections can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals may be either identified or created by intercrossing several different parents. The best plants may be selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population, in which further cycles of selection may be continued.
Backcross breeding has been used to transfer genes for a simply- and highly-heritable trait into a desirable homozygous cultivar, or inbred line, which is the recurrent parent. The source of the trait to be transferred is the “donor parent.” The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar), and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected, and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent and the desirable trait transferred from the donor parent. During backcross breeding, progeny plants comprising the desired phenotype are typically selected at each generation. Where appropriate, progeny plants may also be selected for the presence of molecular markers; e.g., genetic marker alleles and isozyme markers.
A “single-seed descent procedure” refers to the planting of a segregating population, followed by harvesting a sample of one seed per resulting plant, and using the harvested one-seed sample to plant the next generation. When the population has been advanced from the F2 generation to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines each generation, due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed.
In a multiple-seed procedure, breeders commonly harvest seeds from each plant in a population and thresh them together to form a bulk. Part of the bulk is used to plant the next generation, and part is put in reserve. This procedure has been referred to as modified single-seed descent. The multiple-seed procedure has been used to save labor involved in the harvest. It is considerably faster to remove seeds with a machine, than to remove one seed from each by hand for the single-seed procedure. The multiple-seed procedure also makes it possible to plant the same number of seeds of a population for each generation of inbreeding. Enough seeds are harvested to compensate for the number of plants that did not germinate or produce seed.
One set of traits that may be of interest to an oilseed plant breeder are oil traits (e.g., yield and composition). This is in large part due to the fact that vegetable-derived oils have gradually replaced animal-derived oils and fats as the major source of dietary fat intake. However, saturated fat intake in most industrialized nations has remained at about 15% to 20% of total caloric consumption. In efforts to promote healthier lifestyles, the United States Department of Agriculture (USDA) has recently recommended that saturated fats make up less than 10% of daily caloric intake. To facilitate consumer awareness, current labeling guidelines issued by the USDA now require total saturated fatty acid levels be less than 1.0 g per 14 g serving to receive the “low-sat” label and less than 0.5 g per 14 g serving to receive the “no-sat” label. This means that the saturated fatty acid content of plant oils needs to be less than 7% and 3.5% to receive the “low-sat” or “no-sat” label, respectively. Since issuance of these guidelines, there has been a surge in consumer demand for “low-sat” and “no-sat” oils. To date, this demand has been met principally with canola oil, and to a much lesser degree with sunflower and safflower oils.
In addition to direct human consumption, vegetable oil has added value for livestock feed, due to its higher energy density and is also increasingly used as a primary source for biodiesel production, particularly in Europe. Vegetable oils with high oleic acid (a monounsaturated fatty acid), and/or low levels of saturate fatty acids, provide considerable health and cooking benefits when compared to saturated and polyunsaturated fatty acids. Kinney et al. (2002) Biochem. Soc. Trans. 30:1099-103; White and Weber (2003) “Lipids of the kernel,” in Corn: Chemistry and Technology. 2nd Ed., Vol. 10, Eds. White & Johnson, American Association of Cereal Chemists, Inc., St. Paul, Minn., pp. 355-95.